It is well known in the art that the cutting speed of the tool bit of a portable power tool, as for example the drill bit of an electric drill, is a function of both the rotational speed of the motor and the diameter of the tool bit. In particular, it will readily be appreciated that for a fixed motor speed, the cutting speed of a drill bit will increase proportionately with increases in the diameter of the drill bit used. Accordingly, prior art electric drills are known to include motor speed control knobs which can be manually adjusted by an operator to set the speed of the motor to the recommended speed for a given bit diameter as provided on a reference table. With prior art electric drills, therefore, it is incumbent upon the operator to remember to adjust the speed of the motor each time the size of the drill bit is changed. Failure to correctly set the speed of the motor can cause less than optimum operation and a consequent premature wearing of the tool bit.
Accordingly, it is the primary object of the present invention to provide a control device for a portable power tool, such as an electric drill, that will automatically adjust the speed of the motor in accordance with changes in the diameter of the tool bit used.
In addition, it is a further object of the present invention to provide a control device for a portable power tool, such as an electric drill, which also includes means for permitting an operator to initially preset the speed of the motor to an optimum reference speed for a particular type of material so that subsequent motor speed changes caused by changes in bit size are made relative to the preset reference speed.